While the ACGME regulates the vast majority of post-graduate residencies/fellowships, there are a few exceptions. One of the notable ones is the rapidly evolving Kidney Transplant fellowship. Previously, nephrologists were certified as members of the kidney transplant community based on their experience, having met various criteria that were based primarily on past experience with kidney transplant patients; there were not official training programs in existence.
Since 1998, however, the ASN and the American Society of Transplantation (AST) began a joint effort to standardize the training of transplant nephrologists. Because the number of graduates of such programs is relatively small (only 29 in all of 2008, according to the most recent issue of Kidney News), they are not regulated by ACGME. However, there is presently an AST Accreditation Committee which has been charged with approving transplant fellowship programs as sanctioned training programs. I am not sure if it is currently absolutely essential to be trained under an officially licensed program in order to practice as a transplant nephrologist, but my guess is that eventually it will become the law of the land.
For a list of all the 49 AST-accredited transplant nephrology fellowships, click here.